The present invention relates to the technical domains of analysis of products by gas-chromatography.
The principle of gas chromatography is based on the separation of at least two different compounds coming within the composition of a product, in a chromatography column where levels of balance and of separation are established on a porous substrate included in the column.
The apparatus for carrying out such a principle comprise an injector of the product having to be analyzed, a chromatography column placed in an oven with adjustable temperature, a detector of the gaseous phases successively separated and corresponding to the different components of the product and a recorder coupled with the detector.
Apparatus of the above type are satisfactory and enable a chromatogram to be obtained which corresponds to the different vapour phases successively separated.
However, very often, such chromatograms can be validly interpreted in practice only insofar as it is possible to identify the peaks of the chromatogram in relation with the separated vapour phases.
There is therefore a need to be able to withdraw at the outlet of a chromatograph, at least partially, the successively separated vapour phases, so as to be able to proceed on these phases with analyses of identification in order to determine the structure thereof.
The technique heretofore recommended to this end is generally called preparative gas-chromatography and consists in collecting at the outlet of the chromatograph, at least part of each separated gaseous phase.
Preparative gas-chromatography includes a trapping phase consisting in collecting all or part of a separated vapour phase in order to have available, after condensation, a pure standard substance for reference or identification during subsequent analysis work.
For the trapping technique to respond to these objectives, it must be able to furnish products of high purity, capable of being recovered even in a small quantity, being given that it is more and more necessary to work on small samples, due to the cost of the reagents and of the products having to be analyzed, their toxicity or the necessity of short response times which can generally be attained only by means of small-section chromatography columns enabling the finest resolutions to be obtained.
Such a technique must also be able to ensure a certain non-polluted confinement of a small, even minute, quantity of recovered condensate, whilst allowing easy recovery thereof.
Such a technique must also furnish a good trapping yield, in order to limit the successive operations of separation, when the vapour phase to be separated enters in a very small proportion in a product to be analyzed which is available only in a small quantity.
In order to carry out such a trapping technique satisfactorily, it is consequently important to be able to ensure, under optimum conditions and as quickly as possible, the withdrawal of all or part of a separated gaseous phase delivered by the outlet of the chromatograph, and to ensure condensation thereof under good conditions, without risk of pollution.
To carry out such a technique, the prior art proposes several methods.
The first consists in placing, by hand, at the outlet of the chromatograph, a tapered straight tube open at the two ends to recover the separated vapour phase.
This technique is not precise and does not allow a suitable confinement with a good yield, due to the shape of the tube.
To improve this technique, the prior art has also recommended to mount on the outlet of a chromatograph, a valve with multiple outlets which are each connected by a pipe to a recovery tube, most often subjected to a forced cooling by an installation which is always cumbersome and sometimes complex. The valve is controlled in relation with the chromatogram, so as to direct each successively separated vapour phase towards a different recovery tube.
However, this technique, although clearly preferable to the preceding one, presents numerous drawbacks.
The imperative tightness which the valve must present cannot be obtained for certain over a period of time, due, mainly, to the expansions which are imposed thereon by the relatively high temperature of the separated vapour phases delivered by the chromatograph.
These problems of tightness are frequently the cause of pollutions of the separated vapour phases by the ambient medium. The obtaining of condensates of high purity and in small quantity is therefore random.
An additional factor of pollution resides in the fact that a fraction of the previously separated vapour phase distributed always remains in the valve body and that this residual fraction consequently constitutes a source of pollution for the following separated vapour phase.
Another source of pollution resides in the fact that the control of the valve induces, within the column of the chromatograph, successive variations in pressure which disturb the operation of the latter, to the point of altering the capacities of separation or of resolution.
Another drawback of this technique comes from the fact that it would be necessary to construct the valve in a material which is inert with respect to a large number of products in order to avoid any risk of oxidation or corrosion in time.
In an attempt to overcome certain of the drawbacks of the above technique, it has been proposed to eliminate the single distribution valve for all the circuits leading to the recovery tubes and to allocate one valve to each of them. In such a case, the different recovery circuits are connected at a common point to the outlet of the chromatograph.
Such a technique may be of interest as it eliminates the risks of pollution by the residual fractions remaining inside the single valve. However, the same other drawbacks as those set forth hereinabove must be imputed to such a trapping installation.
In an attempt to solve the problem raised, the prior art further proposes another method consisting in fitting a hollow needle at the outlet of a chromatograph, for example in relation with the detector. This needle is made to pass through a vented septum obturating a recovery tube subjected to cooling to ensure condensation of the separated gaseous phase which it is adapted to receive.
This method presents a certain number of appreciable drawbacks.
Firstly, the hollow needle is directed downwardly, in an orientation favourable to pierce the septum of each tube. Such an orientation is not favourable to the recovery of a gaseous phase.
In all cases, the needle is disposed in the ambient medium, i.e. most often in the open air, without being heated. The circulation of a gaseous phase may then produce, particularly for the heaviest compounds, an internal condensation leaving traces which pollute the gaseous phase subsequently separated.
Furthermore, the tubes equipped with the vented septum generally comprise one or more internal baffles provided to facilitate the condensation of the separated gaseous phase. The presence of these baffles represents an obstacle which is sometimes insurmountable for the direct withdrawal of the condensate.
The prior art techniques recommended are therefore not able to respond positively to the objectives having to be taken into account and which are:
the possibility of confinement of the condensates, even in a very small quantity,
the absence of pollution of the trapped condensates,
the possibility of easy withdrawal of the condensate, even obtained in a small quantity,
the obtaining of a good trapping yield,
the absence of a complex, expensive and delicate installation to ensure cooling of each separated gaseous phase.
It is precisely an object of the invention to propose a novel trapping tube, adapted to be fitted to the outlet of a gas chromatograph and of which the design responds to the objects set forth hereinabove, employing only a simple structure easy to produce, of low cost and adapted to be rendered automatic.
To attain this objective, the trapping tube according to the invention is characterized in that it is constituted by a shaped tube comprising:
a first section or zone of condensation of the separated gaseous phase delivered by the outlet of the chromatograph,
a second section or zone of confinement of the condensate of the separated gaseous phase,
and a third section or zone of evacuation of the vector gas of the separated gaseous phase.
It is a further object of the invention also to propose, for the support and presentation of trapping tubes, a device for manual, semi-automatic or totally automatic operation then synchronized with the operation of the phase detector associated with a gas chromagraph.
Such a device for supporting and presenting trapping tubes is characterized in that it comprises:
a fixed support,
a mobile plate mounted on the support,
a motor member for driving the plate step by step in front of the outlet of the chromatograph,
n carriages for supporting and presenting n trapping tubes each mounted on the plate via a slideway extending parallel to the axis of the outlet when the corresponding carriage is in position of presentation,
means for controlling, on the one hand, the individual displacement of each carriage placed in position of presentation, in the direction of the outlet of the chromatograph to ensure placing of the tube and outlet into relationship and, on the other hand, the automatic return of said carriage into standby position.